The filter cartridge of the present invention is of the type as described in U.S. Pat. No. 5,873,967 wherein there is described a vacuum filter device adapted to connect to a pair of tubes with the upper one of the tubes containing a sample liquid medium to be filtered and the other tube or other entities receiving the filtrate passing through the filter. It is important with that filter device that the tube containing the sample liquid medium always be connected to a specific side of the filter as it will only operate in that position due to the construction of the filter cartridge. It is further pointed out that these filter cartridges are constructed for one-time use and after a liquid medium placed in a tube or other entities has been filtered the cartridge is discarded as the filter element cannot be replaced. The filter element is a porous membrane which is welded at its periphery to a plate prior to bonding the two holders of the cartridge together. The filter membrane is constructed from suitable polymeric materials such as mixed esters of cellulose, cellulose acetate, polycarbonate, polyvinylydene fluoride, polytetrafluoroethalene, nylon, polypropylene, polyethylene and other such polymeric substances. Because the filter membrane is welded it is not possible to re-use the entire cartridge after a single use as the porous membrane becomes saturated with trapped particles of the sample fluid medium that was filtered. It is also not possible to use a paper membrane with this filter cartridge.
Because of the welding of the filter membrane the assembly of the cartridge requires a further step in its construction and is therefore not simple in construction and assembly. The holders of the cartridge are also bonded in a back-to-back relationship by a welding technique such as ultrasonic welding to form an integral body. All of these steps in the assembly of the cartridge and associated welding equipment add to the cost of the cartridge which will eventually be discarded after a one-time use.
With these types of cartridges, a vacuum port is provided whereby a vacuum can be applied to the bottom tube whereby the sample liquid medium to be filtered in the upper tube is drawn through the filter. An air vent is provided in the cartridge to permit the ingress of air into the upper tube containing the sample liquid medium. As described in the referenced patent, injection molding methods generally provide the greatest dimensional control of shape with plastic parts. To apply conventional molding techniques for the fabrication of the cartridge would be desirable if one could mold a passageway into the wall of a cartridge to dimensions of 0.015 inches or less. However, because there is deformation caused by flashing when the molded parts are separated, this is not feasible. As described in that patent, this is because as the molten plastic enters the mould cavity the pin used to create the passageway would deflect leading to fatigue and breakage. Also, for the pin to seal off against the other wall of the cavity, the sealing end of the pin will be peened over in time leading to flashing. Flashing is an uncontrollable, undesirable migration of plastic, which in this example would lead to filling and dimensionally distorting the venting passageway. The fact that the venting passageway is dimensioned at 0.015 inches or less provides significant advantages in that the filtration device maintains its liquid-tight capabilities while air is admitted in the upper tube without employing an additional membrane covering the venting passageway to prevent solution from leaking out of the device during normal use. With the prior art this passageway is formed by a more expensive technique than would be the case with injection molding.